A Detroit man has been arrested after buying explosives from an undercover agent, amassing weapons and talking about carrying out an attack, federal authorities said.

A criminal complaint charges Sebastian Gregerson, 29, with unregistered possession of a destructive device and unlicensed receipt of explosive materials. He made an initial appearance Monday in U.S. District Court and is being held pending a Thursday detention hearing.

The Detroit News, which first reported the allegations, said two of Gregerson's supporters declined to comment outside of court. The Associated Press sent an email Tuesday seeking comment from Gregerson's court-appointed lawyer, Miriam Siefer.

The complaint chronicles a counterterrorism investigation that began in April 2015 following a tip that Gregerson, who the FBI said also is known as Abdurrahman bin Mikaayl, had weapons. The FBI said he bought grenades on Sunday from the undercover agent in Monroe and was arrested.

The complaint said Gregerson earlier purchased "an arsenal of weapons, ammunition, tactical gear and tactical training materials." It doesn't say whether Gregerson was planning an attack or why he was purchasing explosives. The U.S. attorney's office in Detroit hasn't elaborated.

In June, the FBI said Gregerson bought road spikes online that can be used to slow or disable vehicles. Other purchases included tactical training items, including handguns and dummy rounds.

"The purchase of training versions of these weapons makes it unlikely that the weapons were purchased for recreational use, such as hunting," the complaint states.